github.com/kubiko/snapd@v0.0.0-20201013125620-d4f3094d9ddf/cmd/snap-update-ns/common.go (about)

     1  // -*- Mode: Go; indent-tabs-mode: t -*-
     2  
     3  /*
     4   * Copyright (C) 2019 Canonical Ltd
     5   *
     6   * This program is free software: you can redistribute it and/or modify
     7   * it under the terms of the GNU General Public License version 3 as
     8   * published by the Free Software Foundation.
     9   *
    10   * This program is distributed in the hope that it will be useful,
    11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
    12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    13   * GNU General Public License for more details.
    14   *
    15   * You should have received a copy of the GNU General Public License
    16   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
    17   *
    18   */
    19  
    20  package main
    21  
    22  import (
    23  	"fmt"
    24  
    25  	"github.com/snapcore/snapd/cmd/snaplock"
    26  	"github.com/snapcore/snapd/logger"
    27  	"github.com/snapcore/snapd/osutil"
    28  	"github.com/snapcore/snapd/sandbox/cgroup"
    29  )
    30  
    31  type CommonProfileUpdateContext struct {
    32  	// instanceName is the name of the snap instance to update.
    33  	instanceName string
    34  
    35  	// fromSnapConfine indicates that the update is triggered by snap-confine
    36  	// and not from snapd. When set, snap-confine is still constructing the user
    37  	// mount namespace and is delegating mount profile application to snap-update-ns.
    38  	fromSnapConfine bool
    39  
    40  	currentProfilePath string
    41  	desiredProfilePath string
    42  }
    43  
    44  // InstanceName returns the snap instance name being updated.
    45  func (upCtx *CommonProfileUpdateContext) InstanceName() string {
    46  	return upCtx.instanceName
    47  }
    48  
    49  // Lock acquires locks / freezes needed to synchronize mount namespace changes.
    50  func (upCtx *CommonProfileUpdateContext) Lock() (func(), error) {
    51  	instanceName := upCtx.instanceName
    52  
    53  	// Lock the mount namespace so that any concurrently attempted invocations
    54  	// of snap-confine are synchronized and will see consistent state.
    55  	lock, err := snaplock.OpenLock(instanceName)
    56  	if err != nil {
    57  		return nil, fmt.Errorf("cannot open lock file for mount namespace of snap %q: %s", instanceName, err)
    58  	}
    59  
    60  	logger.Debugf("locking mount namespace of snap %q", instanceName)
    61  	if upCtx.fromSnapConfine {
    62  		// When --from-snap-confine is passed then we just ensure that the
    63  		// namespace is locked. This is used by snap-confine to use
    64  		// snap-update-ns to apply mount profiles.
    65  		if err := lock.TryLock(); err != osutil.ErrAlreadyLocked {
    66  			// If we managed to grab the lock we should drop it.
    67  			lock.Close()
    68  			return nil, fmt.Errorf("mount namespace of snap %q is not locked but --from-snap-confine was used", instanceName)
    69  		}
    70  	} else {
    71  		if err := lock.Lock(); err != nil {
    72  			return nil, fmt.Errorf("cannot lock mount namespace of snap %q: %s", instanceName, err)
    73  		}
    74  	}
    75  
    76  	// Freeze the mount namespace and unfreeze it later. This lets us perform
    77  	// modifications without snap processes attempting to construct
    78  	// symlinks or perform other malicious activity (such as attempting to
    79  	// introduce a symlink that would cause us to mount something other
    80  	// than what we expected).
    81  	logger.Debugf("freezing processes of snap %q", instanceName)
    82  	if err := cgroup.FreezeSnapProcesses(instanceName); err != nil {
    83  		// If we cannot freeze the processes we should drop the lock.
    84  		lock.Close()
    85  		return nil, err
    86  	}
    87  
    88  	unlock := func() {
    89  		logger.Debugf("unlocking mount namespace of snap %q", instanceName)
    90  		lock.Close()
    91  		logger.Debugf("thawing processes of snap %q", instanceName)
    92  		cgroup.ThawSnapProcesses(instanceName)
    93  	}
    94  	return unlock, nil
    95  }
    96  
    97  func (upCtx *CommonProfileUpdateContext) Assumptions() *Assumptions {
    98  	return nil
    99  }
   100  
   101  // LoadDesiredProfile loads the desired mount profile.
   102  func (upCtx *CommonProfileUpdateContext) LoadDesiredProfile() (*osutil.MountProfile, error) {
   103  	profile, err := osutil.LoadMountProfile(upCtx.desiredProfilePath)
   104  	if err != nil {
   105  		return nil, fmt.Errorf("cannot load desired mount profile of snap %q: %s", upCtx.instanceName, err)
   106  	}
   107  	return profile, nil
   108  }
   109  
   110  // LoadCurrentProfile loads the current mount profile.
   111  func (upCtx *CommonProfileUpdateContext) LoadCurrentProfile() (*osutil.MountProfile, error) {
   112  	profile, err := osutil.LoadMountProfile(upCtx.currentProfilePath)
   113  	if err != nil {
   114  		return nil, fmt.Errorf("cannot load current mount profile of snap %q: %s", upCtx.instanceName, err)
   115  	}
   116  	return profile, nil
   117  }
   118  
   119  // SaveCurrentProfile saves the current mount profile.
   120  func (upCtx *CommonProfileUpdateContext) SaveCurrentProfile(profile *osutil.MountProfile) error {
   121  	if err := profile.Save(upCtx.currentProfilePath); err != nil {
   122  		return fmt.Errorf("cannot save current mount profile of snap %q: %s", upCtx.instanceName, err)
   123  	}
   124  	return nil
   125  }